1. Field of the Invention
The present invention relates to a nitride semiconductor laser element, and more particularly relates to a nitride semiconductor laser element utilized a diffraction grating layer.
2. Background Information
Today it is considered possible to oscillate a semiconductor laser made from a nitride semiconductor over a wide range of wavelength bands, from the ultraviolet band to red, and such lasers can not only be used in DVDs and other such optical disk systems that allow information to be recorded and reproduced in large capacity and high density, but also hold promise as a light source or the like for laser printers, optical networks, and so on.
A semiconductor laser element generally has a certain energy gap width centered on an energy gap which is determined by the active layer, so the emission wavelength depends on it, and the gain of the light amplification increases. Thus, there are a number of longitudinal l modes with a mode interval depending on the cavity length. In addition to this broad mode distribution, this distribution tends to shift toward the longer wavelength side while it significantly varies according to the output or the drive temperature of the laser element, so when optical communication is performed over a long distance, for example, the transmission rate is different for each mode, and there is scattering.
Because of this or the like, particularly in optical communications and other such applications, a vertical single-mode, single-frequency laser is required, and a DFB (distributed feedback) laser diode has been proposed to obtain a distinct single longitudinal mode emission (see, for example, Japanese Laid-Open Patent Application H8-195530, H9-191153, 2000-223784, 2001-203422 and 2002-131567).
With this DFB laser diode, a diffraction grating layer that reflects light periodically is provided parallel to the active layer in a double hetero structure, and it is believed that light generated in the active layer is periodically reflected by the period of this diffraction grating, and the peaks and valleys of the original light and the reflected light match up and reinforce each other, allowing a laser beam output with a single frequency to be obtained.
With DFB laser diodes proposed in the past, however, the effect of the diffraction grating was still not satisfactory.
For instance, to obtain a satisfactory diffraction grating effect with a DFB laser diode based on gallium nitride, recessed and raised portions have to be precisely formed on the nitride semiconductor laser in order to form the diffraction grating. However, it is difficult to etch at high precision with a semiconductor material based on gallium nitride, and it has been difficult to obtain a diffraction grating that would periodically reflect light generated by the active layer. Also, to obtain a single spectrum peak with a DFB laser diode, it is necessary to provide the refractive index differential more efficiently in the diffraction grating. To this end, it is possible to obtain a refractive index differential by increasing the depth of the recessed and raised portions of the diffraction grating, but the depth of the recessed and raised portions is also difficult to form precisely, and problems in terms of mass production and reproducibility are encountered in forming an effective diffraction grating.
Also, even if these recessed and raised portions can be formed, when a semiconductor is regrown over this diffraction grating, voids may be formed between the recessed and raised portions and the regeown semiconductor layer. If unwanted voids should be generated between the diffraction grating and the overlying regrown semiconductor layer, these voids will end up changing the refractive index, so the light will not be reflected periodically, making it difficult to obtain the effect of the diffraction grating. Also, if voids are formed between the recessed and raised portions and the regrown semiconductor layer, the voltage will be higher when the laser element is driven, and this tendency is more pronounced when the diffraction grating is provided on the p side, so it tends to cause problems in the performance of the laser element.
The Other problem has been that the recessed and raised portion are not produced flat top surface after regrowth process of semiconductor layer, or dislocation occurs from the interface between the recessed and raised portions and the semiconductor layer that embeds the recessed and raised portions. Such problems make it difficult to form the semiconductor layer that embeds the recessed and raised portions so as to ensure good performance of the laser element (laser diode) itself.